I still think it would have been a good idea if Lysithea had Macuil's Crest : not only it would work like Gloucester's but you could have an interesting convo between her and Macuil bonding over their hatred of the Agarthans.

Now how they would gotten it ? The same way they managed to get Sothis's blood to transfer it to Edelgard despite the fact that Sothis died ages ago and back then there was no way to preserve her blood.

And I say this because Sothis' blood is litteraly the only way that Edelgard could possibly form the Crest of Flames.

I do have an interesting theory about Crests though : I theorize that all Crests are born from a mutation of the Crest of Flames and said mutation is due to the Nabateans'unique physiology meaning that in theory each Nabatean has a unique Crest.
Would explain why none of them have the Crest of flames or why Flayn doesn't have Cichol's Crest